Pre-filled hypodermic syringes provide several advantages for the home-use market. These advantages include that pre-filled syringes may be prepared for each medicament with exactly the required dosage. Further, they are easily operated, by merely advancing the stopper of the syringe. Aside from the costs of the particular medication used, pre-filled syringes are also economically manufactured. Consequently, all these advantages make pre-filled syringes commercially appealing.
Nevertheless, pre-filled syringes also have a significant drawback in the marketplace. Specifically, many users are either frightened by an exposed needle or feel they are inherently incapable of performing an injection. Because of aversions to exposed needles, as well as health and safety issues that may be involved, various types of injectors and other devices have been developed for the specific purpose of concealing needles from the user and automating the injection task to assist the user in performing the injection.
In order to inject a fluid medicament into a patient when using a hypodermic syringe, generally three separate and distinct tasks must be performed. These are: 1) insertion of the needle into the patient; 2) injection of the fluid medicament from the syringe into the patient; and 3) withdrawal of the needle after the injection has been completed. For each task, the magnitude and direction of forces on the syringe, as well as the location of their application, are different from the other tasks. For instance, compare the task of inserting the needle, with the task of injecting the fluid medicament. Insertion of the needle requires that only minimal forces be applied on the syringe, and that they be applied for only a very short period of time. On the other hand, injection of the medicament requires a much greater force be applied. Further, this force must be applied on the plunger of the syringe for what will typically be a relatively longer period of time. In comparison with both of these tasks, needle withdrawal requires the application of a force in the opposite direction. These, and other similar considerations, become important when the injection process is to be automated.
Springs for generating forces on a syringe in an automated process have been used heretofore for various purposes. A characteristic of springs, however, is that the magnitude and direction of a spring force are not variable. Consequently, springs do not lend themselves to multi-tasking operations. This limitation is particularly notable in a syringe injection, which requires precise control of sequential forces of different magnitude (needle insertion and medicament injection). This limitation can be particularly problematic where it may be desirable to use the same device, at different times, to inject different medications with different fluid viscosities.
In addition to these mechanical considerations, the design of an autoinjector requires user-friendly considerations. In particular, it is desirable that the injection needle of a syringe be operationally concealed from the view of a user. Preferably, this concealment is maintained before, during and after an injection procedure. Further, it is desirable that operation of the syringe be limited to only those times when the syringe is properly positioned for an injection.
Accordingly, an improved autoinjector apparatus is needed.